Several types of data storage devices, such as flash memory devices, store data in cells of non-volatile media. A physical property of each cell, such as a stored charge, voltage, material phase, electrical resistance, magnetization, or the like, is alterable to encode data. A cell's physical property may be variable across a range, which may be divided into discrete states, so that different states correspond to different data values. Sensing whether the cell's physical property satisfies one or more read thresholds (voltage thresholds, resistivity thresholds, or the like) within its range determines the cell's state, thus allowing recovery of a stored data value.
The data-encoding physical property of a cell may vary over time due to cell damage, charge leakage, temperature effects, disturbances from nearby cells, or the like. An error may occur if the cell moves from one state into an adjacent state. As storage density increases, feature size shrinks, making the cells more susceptible to such disturbances, and increasing error rates. Encoding data for storage with error correcting codes may compensate for such errors.
Soft-decision error correcting code (ECC) decoders achieve high error correction performance by operating on “soft” data, such as log-likelihood ratio (LLR) inputs, which indicate a data value and a probability that the data value is correct, unlike hard-decision decoders, which operate on non-probabilistic “hard” data. Using additional read thresholds for “soft sensing” may provide additional “soft read information” about a cell, which can be mapped to LLR values for a soft-decision ECC decoder. For example, a data value from a cell is more likely correct if soft read information indicates that the cell's physical property is in the middle of a state, and is more likely in error if soft read information indicates that the cell's physical property is near a boundary between states, where it may have moved from an adjacent state.
A mapping from soft read information to LLR values may provide accurate probabilistic data to an ECC decoder under certain conditions. However, leakage and other disturbances may increase with age, as cells are used over time. Therefore, a single, static mapping from soft read information to LLR values may provide less accurate probabilistic data to an ECC decoder, over time.